Synthetic rutile and its preparation



United States Patent SYNTHETIC RUTILE AND ITS PREPARATION Leon Merker, New York, N. Y., assignor to National Lead gompany, New York, N. Y., a corporation of New ersey No Drawing. Application January 22, 1953, Serial No. 332,751

2 Claims. (Cl. 106-42) The present invention relates to rutile single crystal boule modified with respect to color and certain other physical properties, and to methods for the production of such boules.

Rutile is one of the three known crystal modifications of titanium dioxide. When substantially pure, a massive single crystal of rutile has gem-like properties with a very light straw color and reflectance, refraction and brilliance greater than that of a diamond. By means of the present invention rutile single crystals may be produced which have a blue color ranging from a bluish white to a blue black color.

The principal object of the present invention is the preparation of rutile single crystals having a blue color. Another object of this invention is a method for producing a rutile single crystal boule which possesses a color within the range from bluish white to blue black. These and other objects of the present invention will become apparent from the following more complete description.

In its broadest aspects this invention contemplates a rutile single crystal containing a small amount of Ian oxidic compound of beryllium in solid solution and characterized by having a blue color prepared according to the method of the present invention by progressively fusing a mixture of finely divided TiOz containing a small amount of an oxidic compound of beryllium in a flame carrying an excess of an oxidizing gas and thereafter subjecting the boule so formed to an oxidizing treatment.

In this description of the present invention the word boule is used in its currently accepted meaning to denote a characteristic shape or form of an artificially prepared massive single crystal having a rounded end, or meniscus, a more or less rod-like body portion and a tapering, end, giving an overall appearance in profile of an almond, or carrot.

The general method to which the present invention relates is that described in application Serial No. 54,562 filed October 14, 1948, now abandoned, for the preparation of substantially pure rutile single crystal boules.

According to the preferred method of the present invention, there is employed as starting material a substantially pure TiOz which is very fine, fairly uniform and possesses an open structure with units capable of being rapidly melted. A TiOz having an ultimate unit particle size of approximately 0.1 micron has proved especially satisfactory. In general, material having an average particle size above about 5 microns should be avoided because such particles do not satisfactorily fuse under the conditions of the invention. A preferred method for preparing a TiOz starting material especially adapted for the production of rutile single crystal boules is described in U. S. Patent No. 2,521,392, wherein the double salt, ammonium-titanium-sulphate is calcined until all of the ammonia'and sulfate has been removed by volatilization.

According to the present invention a small amount of the coloring oxide may be mixed with the TiOz starting "'ice material in any convenient manner, for instance, by dry mixing. A more uniform mixture and therefore a preferred mixture may be prepared by adding compounds of the coloring agent as a powder, slurry, or solution, to the double salt ammonium-titanium-sulfate prior to calcination. The amount of coloring agent to be added to the TiOz starting material is very small and should not exceed about 1% calculated as the metal oxide based on the weight of the TiO2. This invention covers adding an effective amount of beryllium oxide, or any oxidic compound of beryllium which forms beryllium oxide at the temperatures employed, up to about 1% of the mixture by Weight. The depth of color obtained in the reoxi dized boule is dependent upon the amount of oxidic coloring agent employed. It has been found that the more coloring agent added, the darker the color obtained after the reoxidization step. As previously stated, the color of the boule produced in the furnace is substantially black in color. Upon reoxidizing the boule a lighter color is obtained. It has been found that a bluish white color is obtained upon oxidizing the boule instead of the normally strawwhite color of rutile boule when about .001 to .005% are added to the rutile feed material. From these amounts up to about 1% of the colored oxides may be added, the larger amounts, i. e. from 0.1 to 1% produces very dark blue to blue black colors upon oxidation. The rutile single crystal boule of this invention consisting of titanium dioxide and beryllium oxide may, and often will, contain small amounts of various compatible substances which may enter as impurities in the raw materials. These substances may be only of a character and may be present only in amount so that the single crystal structure of the boule and its color are not substantially affected.

In progressively fusing the starting mixture of TiOa and a compound of beryllium oxide there should be employed a flame which carries an excess of an oxidizing gas, preferably oxygen. Preferably the fusion is carried out at a temperature of from 1830 C. to 1870 C. according to the method described in application Serial No. 54,562, supra, in which the particles of the starting mixture are entrained in a stream of oxygen in a tube constituting the center of a burner consisting of three concentric tubes. In this burner hydrogen is introduced to the flame through the intermediate tube and additional oxygen in excess over the amount required to combine with the hydrogen is introduced through the outer tube. Preferably, the ratio of oxygen to hydrogen by volume should be about 1:1 and should be not less than about 8.5 parts of oxygen to 9.5 parts hydrogen by volume.

The excess of oxygen in the flame is necessary because at elevated temperatures, titanium dioxide gives up oxygen and is converted to lower, or suboxides of titanium, e. g. TiaOs. This loss of oxygen proceeds with con siderable rapidity under conditions produced by an oxyhydrogen flame wherein the TiOz is fused and the loss is accelerated when the frame is neutral or reducing. Under such conditions the formation of lower, or suboxides of titanium is so great that mixtures of small crystals of various oxides of titanium are produced rather than a single crystal boule.

Even when operating with an excess of oxygen the boule as produced has a generally deep blue-black color indicative of some deficiency in oxygen. Therefore, in order to produce a boule of a lighter blue color, the blueblack boule has to be reoxidized.

The reoxidation of the boule is preferably carried out by heating the blue-black boule in an oxidizing atmosphere, for instance, air or oxygen. The temperature of the treatment should be within the range from about 650 C. to about 1400 C. It has been found that at ternperatures to any extent below 650 C. oxygen Will not be appreciably incorporated into the rutile single crystal. Preferably the oxidation should be carried out at about 1000 C. to 1300 C. The heating should be continued until the boule obtains the maximum degree of lightness of color.

As produced, rutile boule containing the oxidic compound of beryllium has a more or less frosted outer surface. When split, the interior surfaces of the pieces are vitreous and shiny. They may be cut :and polished to form beautiful gems possessing a clear blue color ranging from bluish white to blue black. In order to show the effects of the various amounts of coloring agent upon the depth of color, the following table is presented showing the various percentage ranges which produce various depths of blue color.

0.001% to 0.005% to Beryllium oxide bluish white light blue to very dark to 20 dark blue. blue black.

was aligned axially with such flame; the material melted in the flame being crystallized progressively by moving the support and flame apart axially of the flame. The blue black boules produced by the flame were then subjected to a subsequent oxidizing treatment at temperature from 1000 C. to 1300 C. to produce the desired color.

From the. above description and by the examples presented, it has clearly been shown that blue rutile single crystal boules may be produced by adding small amounts of oxidic compound of beryllium. The depth of color produced by the subsequent oxidation step is dependent upon the amount of coloring agent employed.

While this invention has been described and illustrated by the examples shown, it is not intended to be strictly limited thereto and other modifications and variations may be employed within the scope of the following claims.

I claim:

1. A blue single crystal of synthetic rutile composition consisting of titania and from about 0.001% to 0.1% by weight beryllium oxide.

2. A blue single crystal of synthetic rutile composition consisting of titania and an amount from about 0.005% to about 0.1% by weight beryllium oxide.

References Cited in the file of this patent UNITED STATES PATENTS 

1. A BLUE SINGLE CRYSTAL OF SYNTHETIC RUTILE COMPOSITION CONSISTING OF TITANIA AND FROM ABOUT 0.001% TO 0.1 BY WEIGHT BERYLLIUM OXIDE. 